This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Recycling is the process of collecting and processing materials that would otherwise be thrown away as trash, and turning them into new products. Recycling has benefits for communities and for the environment, since it reduces the amount of waste sent to landfills and incinerators, conserves natural resources, increases economic security by tapping a domestic source of materials, prevents pollution by reducing the need to collect new raw materials, and saves energy. After collection, recyclables are generally sent to a material recovery facility to be sorted, cleaned, and processed into materials that can be used in manufacturing.
The recycling of aluminum (Al) scrap is a very attractive proposition in that up to 95% of the energy costs associated with manufacturing can be saved when compared with the laborious extraction of the more costly primary aluminum. Primary aluminum is defined as aluminum originating from aluminum-enriched ore, such as bauxite. At the same time, the demand for aluminum is steadily increasing in markets, such as car manufacturing, because of its lightweight properties. As a result, there are certain economies available to the aluminum industry by developing a well-planned yet simple recycling plan or system. The use of recycled material would be a less expensive metal resource than a primary source of aluminum. As the amount of aluminum sold to the automotive industry (and other industries) increases, it will become increasingly necessary to use recycled aluminum to supplement the availability of primary aluminum.
Correspondingly, it is particularly desirable to efficiently separate aluminum scrap metals into alloy families, since mixed aluminum scrap of the same alloy family is worth much more than that of indiscriminately mixed alloys. For example, in the blending methods used to recycle aluminum, any quantity of scrap composed of similar, or the same, alloys and of consistent quality, has more value than scrap consisting of mixed aluminum alloys. Within such aluminum alloys, aluminum will always be the bulk of the material. However, constituents such as copper, magnesium, silicon, iron, chromium, zinc, manganese, and other alloy elements provide a range of properties to alloyed aluminum and provide a means to distinguish one aluminum alloy from the other.
The Aluminum Association is the authority that defines the allowable limits for aluminum alloy chemical composition. The data for the aluminum wrought alloy chemical compositions is published by the Aluminum Association in “International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys,” which was updated in January 2015, and which is incorporated by reference herein. In general, according to the Aluminum Association, the 1xxx series of wrought aluminum alloys is composed essentially of pure aluminum with a minimum 99% aluminum content by weight; the 2xxx series is wrought aluminum principally alloyed with copper (Cu); the 3xxx series is wrought aluminum principally alloyed with manganese (Mn); the 4xxx series is wrought aluminum alloyed with silicon (Si); the 5xxx series is wrought aluminum primarily alloyed with magnesium (Mg); the 6xxx series is wrought aluminum principally alloyed with magnesium and silicon; the 7xxx series is wrought aluminum primarily alloyed with zinc (Zn); and the 8xxx series is a miscellaneous category.
The Aluminum Association also has a similar document for cast alloys. The 1xx series of cast aluminum alloys is composed essentially of pure aluminum with a minimum 99% aluminum content by weight; the 2xx series is cast aluminum principally alloyed with copper; the 3xx series is cast aluminum principally alloyed with silicon plus copper and/or magnesium; the 4xx series is cast aluminum principally alloyed with silicon; the 5xx series is cast aluminum principally alloyed with magnesium; the 6xx series is an unused series; the 7xx series is cast aluminum principally alloyed with zinc; the 8xx series is cast aluminum principally alloyed with tin; and the 9xx series is cast aluminum alloyed with other elements. Examples of cast alloys utilized for automotive parts include 380, 384, 356, 360, and 319. For example, recycled cast alloys 380 and 384 can be used to manufacture vehicle engine blocks, transmission cases, etc. Recycled cast alloy 356 can be used to manufacture aluminum alloy wheels. And, recycled cast alloy 319 can be used to manufacture transmission blocks.
Generally speaking, wrought aluminum alloys have a higher magnesium concentration than cast aluminum alloys, and cast aluminum alloys have a higher silicon concentration than wrought aluminum alloys.
Furthermore, the presence of commingled pieces of different alloys in a body of scrap limits the ability of the scrap to be usefully recycled, unless the different alloys (or, at least, alloys belonging to different compositional families such as those designated by the Aluminum Association) can be separated prior to re-melting. This is because, when commingled scrap of plural different alloy compositions or composition families is re-melted, the resultant molten mixture contains proportions of the principle alloy and elements (or the different compositions) that are too high to satisfy the compositional limitations required in any particular commercial alloy.
Moreover, as evidenced by the production and sale of the Ford F-150 pickup having a considerable increase in its body and frame parts composed of aluminum instead of steel, it is additionally desirable to recycle sheet metal scrap (e.g., wrought aluminum of certain alloy compositions), including that generated in the manufacture of automotive components from sheet aluminum. Recycling of the scrap involves re-melting the scrap to provide a body of molten metal that can be cast and/or rolled into useful aluminum parts for further production of such vehicles. However, automotive manufacturing scrap (and metal scrap from other sources such as airplanes and commercial and household appliances) often includes a mixture of scrap pieces of wrought and cast pieces and/or two or more aluminum alloys differing substantially from each other in composition. Thus, those skilled in the aluminum alloy art will appreciate the difficulties of separating aluminum alloys, especially alloys that have been worked, such as cast, forged, extruded, rolled, and generally wrought alloys, into a reusable or recyclable worked product.
Two examples of aluminum alloys used in automotive manufacturing are 5052 and 6061 series alloys; their respective chemical compositions are shown in FIG. 3. Four examples of cast aluminum alloys include 319, 383, 380, and 360; the chemical composition of cast alloy 380 is shown in FIG. 3, while the compositions of the others are well-known and publicly available. Because wrought and cast aluminum alloys differ by chemical composition, a supposedly desired method for sorting these alloys at a high throughput rate would be with a technology that directly measures chemical composition for each piece. However, there are no cost-effective methods to sort aluminum alloys into cast and wrought with direct chemical composition measurement in a cost-effective fashion that makes the process profitable.
While it would therefore be beneficial to be able to sort a mass or body of aluminum scrap containing a heterogeneous mixture of pieces of different alloys, to separate the different alloy compositions or at least different alloy families before re-melting for recycling, scrap pieces of different aluminum alloy compositions are not ordinarily visually distinguishable from each other. Optically indistinguishable metals (especially alloys of the same metal) are difficult to sort. For example, it is possible but not easy to manually separate and identify small pieces of cast from wrought aluminum or to spot zinc or steel attachments encapsulated in aluminum. There also is the problem that color sorting is nearly impossible for identically colored materials, such as the all-gray metals of aluminum alloys, zinc, and lead.
Currently, the only existing technology which separates cast from wrought in a cost-effective fashion is an x-ray transmission technology. Because cast is heavier than wrought due to the higher silicon concentration, the cast alloys are denser than the wrought alloys. The x-ray transmission technology is able to measure the heavier density cast aluminum alloys and then sort the cast from the wrought alloys.
However, this method is not perfect. For example, cast alloys 319 and 383 have a relatively high zinc concentration (e.g., ˜3%), giving these cast alloys their higher respective density. Cast alloy 360 however, has a lower relative zinc concentration (e.g., ˜0.5%), and therefore lower density. The lower density of cast alloy 360 causes the x-ray transmission method to classify this alloy as a wrought alloy and not a cast alloy. Therefore, the x-ray transmission technology does not classify all of the cast alloys correctly due to the large variance in their respective densities. Thus, such cast alloys end up being sorted along with the wrought aluminum alloys, which will result in too much relative silicon in the melted mixture.